In a subsequent step 103, the integral of the battery current over time is determined. Step 103 can be carried out immediately after initiation of the starting operation, or it may also be carried out at any desired time during battery operation. Suitable periods of time over which the integral is determined are obtained in a conventional manner.
The operability of the starter battery of the internal combustion engine in a motor vehicle (or any other system powered primarily by an internal combustion engine) is one of the most important prerequisites for ensuring operating reliability. An internal combustion engine is generally started by an electric starter which draws power for the starting operation from the starter battery. The starter must briefly apply a torque capable of turning over the internal combustion engine at a certain minimum rpm. To do so, a sufficiently high current must flow through the starter or a sufficiently high voltage must be applied to the starter. If the starter battery is very cold, partially discharged or severely aged, its internal resistance may be so high that insufficient current and voltage are available to ensure this starting operation. Such a problem is described in German Patent No. 19705634 C2, for example.
An attempt has been made to determine the charge status of a battery, in particular an automotive lead-acid battery, based on a known initial charge status, e.g., a full charge, by integration of the battery current over time. However, in this case the charge status must be recalibrated at certain intervals because of errors that occur in measuring the current and in current integration and because of battery gassing currents. For example, it is conceivable here to use the open-circuit voltage, which can be determined with a battery with no load connected to it after a lengthy resting phase, in particular lasting for more than four hours, and is proportional to the charge status. However, it has been found that the accuracy of such methods is inversely proportional to the length of the resting phase, i.e., such methods lead to unsatisfactory results in the case of operating cycles with short resting phases such as taxi operation.
An object of the present invention is to improve on the accuracy of methods of determining the charge status of a battery, in particular for operating cycles with short resting phases.
In contrast with known methods of determining the charge status, where current integration and recalibration must be performed, in the method according to the present invention the current integral is not replaced by a calibration value but instead it is corrected by an instantaneously measurable no-load voltage U0 to form a weighted mean. It is also possible with this measure to use short resting phases, in particular resting phases lasting less than four hours, with a battery under no load for correction of the current integral, so that errors occurring in current integration can be minimized relative to the errors occurring in traditional methods, in particular with corresponding operating cycles with short resting phases (especially in taxi operation). Another advantage of the method according to the present invention is that errors in determination of the open-circuit voltage from longer resting pauses have less effect on the charge status determined. Thus on the whole, the accuracy in charge status determination when using current integration methods is improved by the method according to the present invention.
According to a preferred embodiment of the method according to the present invention, the weighted mean is used to determine charge status soc on the basis of an equation in the form:
soc=(1xe2x88x92gew)*soci+gew*socU0xe2x80x83xe2x80x83(1)
where gew is a weighting factor for which the inequality 0xe2x89xa6gewxe2x89xa61 holds. The introduction of such a weighting factor can be handled relatively easily by computing, and the software complexity required to carry out the method according to the present invention can be kept low.
It is especially preferable that the weighting factor is selected on the basis of an equation of the form:
gew=1xe2x88x92exp (xe2x88x92tR/TR)xe2x80x83xe2x80x83(2)
where tR is the duration of a resting phase prior to the measurement, and TR is the time constant of the transient response of no-load voltage U0 until it reaches open-circuit voltage UR of the battery. In conventional terminology, open-circuit voltage UR is understood to be a no-load voltage U0 which is ultimately established after a prolonged period of no load condition and which depends on the battery temperature and its history (charging or discharging before the resting phase).